Help! Poor GSM signal and 4G/LTE after latest OTA update

Hi, I used this s6 (G920F) for more than a year without any issues. Few weeks ago I got an OTA update and since then the phone rarely goes to 4G mode (it almost always used to be in 4G), often stays 'out of coverage' and drains much more juice from the battery. I confirmed the phone is faulty by swapping the SIM over to another phone. This is the currrent config: AP: G920FXXS4DPG6, CP: G920FXXS4DPG3, CSC: G920FOJV3DPF3. Country code XSG, Status: Official, Knox: 0x0.

I already tried these:
Factory reset, Factory reset and wipe in recovery mode. Nil fix.
Emergency recovery/Re-initialise using SmartSwitch. It re-installed the same firmware with the same issues. Nil fix.
Numerous attempts to flash the previous firmware (G920FXXU3DPER_G920FOJV3DPF3_G920FXXU3DPF2_HOME.ta r.md5) using Odin3.11.1 with this result:
<ID:0/005> Added!!
<OSM> Enter CS for MD5..
<OSM> Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/005> Odin engine v(ID:3.1101)..
<ID:0/005> File analysis..
<ID:0/005> SetupConnection..
<ID:0/005> Initialzation..
<ID:0/005> Get PIT for mapping..
<ID:0/005> Firmware update start..
<ID:0/005> SingleDownload.
<ID:0/005> sboot.bin
<ID:0/005> NAND Write Start!!
<ID:0/005> FAIL!
<ID:0/005>
<ID:0/005> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)

Flashed the current firmware using Odin (Passed), wiped in recovery, boot up, the same issues..

Has anyone experienced similar issue? Got any fix? I am out of warranty and have no problem rooting etc. if I must.
My primary intention was to restore the previous stock firmware, but it seems the bootloader will not allow this. Is there any workaround to bypass this block?
Could someone please help? Thanks!

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